Integrated circuit voltage pumps or charge pumps are used in a diverse range of applications where it is required to provide a DC voltage that is numerically larger than any useful or accessible positive or negative DC voltage in a particular application. Many of these applications are related to portable and battery operated equipment such as mobile terminals or electronic or electroacoustical components therefore. One important area of application for voltage pumps is the supply of DC bias voltage to condenser microphones such as miniature microelectromechanical condenser microphones for telecommunication equipment. In this type of microphone application, an integrated circuit voltage pump is often integrated with other types of signal processing and conditioning circuitry such as preamplifiers, I/O interfaces, voltage regulators, A/D converters etc on a common semiconductor die.
The condenser microphone comprises a transducer element comprising a displaceable diaphragm adjacently positioned to a perforated back plate. The distance between the diaphragm and back-plate is often referred to as an airgap height. In operation, a DC bias voltage is applied between the diaphragm and back-plate by an extremely high impedance DC voltage source. The diaphragm and back-plate form plates of a capacitor structure with an intermediary electrical field generated by the DC bias voltage. Due to the extremely high impedance of the DC voltage source, electrical charge on the capacitor structure is essentially kept constant during operation and sound pressure can be detected by amplification of an induced AC or signal voltage that is essentially proportional with sound pressure impinging on the diaphragm.
The condenser microphone may comprise a microelectromechanical (MEMS) transducer element or a conventional condenser transducer element. MEMS based microphones are often fabricated in batched oriented processes by the application semiconductor processing technologies.
It is generally desirable to make the DC output voltage of an integrated circuit voltage pump as accurate as possible over semiconductor processes variations, supply voltage variations and across an operational or target temperature range such as between 0 and 70 degree Celsius.
This is also true for voltage pumps for condenser microphones because an electroacoustical sensitivity of a condenser microphone is directly related to a level of the applied DC bias voltage. However, telecom condenser microphones with integrated circuit voltage pumps are sold in high volumes and at very low prices. As the cost of an integrated circuit is essentially directly related to the area of the semiconductor die, it is important, for the purpose of reducing price, to minimize die area occupied by the voltage pump.
WO 2005/055405 discloses an exemplary prior art integrated circuit voltage pump based on a Dickson converter. The Dickson converter comprises a plurality of cascaded pump stages wherein each pump stage comprises two cascaded semiconductor diodes or diodes with a first pump capacitor disposed in-between the diodes and a second capacitor electrically connected to the cathode of the second diode. Each of the diodes is brought into and out of its forward or conducting mode in an alternating manner by a pair of non-overlapping voltage pulses supplied to the first and second pump capacitors, respectively. Since each diode is connected in series with its respective pump capacitor, variations in the voltage drop across a diode, for example caused by temperature changes or semiconductor process variations, in conducting mode lead to a corresponding change of the pump capacitor voltage. This change of the pump capacitor voltage will be reflected throughout a cascade of pump stages to the DC output voltage of the integrated circuit voltage pump. A diode voltage drop across a diode such as a diode-connected PMOS transistor has a temperature coefficient of about 2 mV per degree C. so that a 40 degrees change of operating temperature leads to a change in diode voltage drop of about 80 mV. This temperature effect is obviously multiplied with the number of individual pump stages and leads to an undesirable and possibly large change of the DC output voltage of the integrated circuit voltage pump across a nominal or desired temperature range.
This and other problems are solved in accordance with the present invention wherein the integrated circuit voltage pump comprises temperature compensation circuitry that eliminates or reduces the effect of the above-described temperature induced changes of the diode voltage drops in conducting mode.